Electric induction furnace

ABSTRACT

An electric induction furnace has an inductor coil formed from a tubular conductor through which cooling water may be flowed, and which is made with an inner layer of corrosion-resisting copper or copper alloy and an outer layer of aluminum or aluminum alloy and which, although oxidizable in air, forms an electrically nonconductive oxide layer eliminating the risk of short-circuiting between the coil convolutions. Material costs are reduced by making the aluminum layer considerably thicker than the copper layer. The two layers are integrated under high mechanical pressure during the manufacture of the conductor, the electrical resistance between the interfaces of the two layers being negligible.

United States Patent 11 1 Andersson et 211.

l 3,900,695 '1451 Aug. 19, 1975 l l ELECTRIC INDUCTION FURNACE [73] Assignee: Allmanna Svenska Elektriska Aktiebolaget, Vasteras. Sweden [22] Filed: May 16, 1974 [21] Appl, No: 470,604

[31)] Foreign Application Priority Data May 18, 1973 Sweden 73117026 [52] US. Cl. 13/26; 174/126 CP; 219/1079 [51] Int. Cl.-' H05B 5/12 [58] Field of Search 13/26, 27, 28; 219/1079;

156] References Cited UNITED STATES PATENTS 1017,4145 1/1962 Hansen 13/27 x Beckius 13/27 Synnestvedt 13/27 Primary limminer Rl N. Envall, .lr. Armrnc v Agcnl, 0r FirmKenyon & Kenyon Reilly Carr & Chapin [57 1 ABSTRACT An electric induction furnace has an inductor coil formed from a tubular conductor through which cooling water may be flowed, and which is made with an inner layer of corrosion-resisting copper or copper alloy and an outer layer of aluminum or aluminum alloy and which, although oxidizable in air, forms an electrically non-conductive oxide layer eliminating the risk of short-circuiting between the coil convolutions, Material costs are reduced by making the aluminum layer considerably thicker than the copper layer. The two layers are integrated under high mechanical pressure during the manufacture of the conductor, the electrical resistance between the interfaces of the two layers being negligible 1 Claim, 3 Drawing Figures PATENTED AUG 1 9 I975 ELECTRIC INDUCTION FURNACE BACKGROUND OF THE INVENTION An electric induction furnace conventionally comprises a crucible for containing a metal melt and an electric induction coil for inductively heating a melt in the crucible. The coil is a coiled tubular conductor through which a cooling liquid can be flowed.

Heretofore, the tubular conductor has been made entirely of copper or possibly a copper alloy. This has been done not only because of the high electrical conductivity of copper, but also because of its resistance to corrosion, the cooling water passed through the tubular conductor sometimes being of a generally corrosive nature.

However, the cost of copper is currently increasing so that such a conventional coil is becoming excessively expensive, making the furnace cost correspondingly undesirably great. Another undesirable feature is that copper easily oxidizes in air. This makes it possible for such a coil to become oxidized on the surface of the tubular conductor, and because the compound thus formed is electrically conductive, such use of copper introduces the risk of short circuits between the convolutions of the inductor coil.

Contrastingly, aluminum and many of its alloys have an electrical conductivity approaching that of copper and are less expensive than a copper or copper alloy. This suggests the possibility of a furnace inductor coil made ofa tubular conductor formed from aluminum or an appropriate aluminum alloy. Although aluminum also rapidly oxidizes in air, its oxides, such as might form on the outside of an inductor coil conductor, provide a dense, electrically non-conducting layer which does not involve the short-circuiting risk referred to above.

Unfortunately, aluminum has the disadvantage that when used as the tubular conductor ofa furnace inductor coil, it can be eroded by the internal flow of the cooling water, or affected by this cooling water in some other way. This disadvantage has heretofore eliminated aluminum from consideration as a substitute for copper or copper alloy in connection with the manufacture of an electric furnace inductor coil.

SUMMARY OF THE INVENTION The object of the present invention is to provide an electric furnace inductor coil having the advantage of copper and aluminum combined.

Briefly, these advantages are combined by providing the furnace with an inductor coil made from a tubular conductor having inner and outer metal layers, the inner layer being a copper or suitable copper alloy and the outer layer being made of some other metal which is substantially free from copper; specifically, the outer layer is made from aluminum or one of the aluminum alloys having high electrical conductivity. This outer layer is made substantially thicker than the inner layer to reduce the materials cost, and therefore carries the majority of the current. the inner layer. with its good corrosion resistance, representing only about 7 to 20 percent of the total metal area of the tubular conductor. This multi-layer tubular conductor may be made by the hydrostatic extrusion method, using conventional techniques, so that the interfaces of the two layers are integrated to a degree resulting in negligible electrical resistance between the two layers, thus substantially eliminating the possibility of coil-overheating due to such resistance should it be present.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings schematically illustrate the presently preferred mode for carrying out this invention, the various figures being as follows:

FIG. I is a vertical section showing conventional tubular product extrusion equipment;

FIG. 2 is a cross section of the conductor; and

FIG. 3 shows the induction furnace using the coil of this invention.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 2, the furnace coil tubular conductor is shown as defining the interior water channel 11 with a round section and having the layer or liner conductor 12 made of copper or a copper alloy, outside of which is the tubular conductor 13 made of aluminum or a suitable aluminum alloy. The inner layer 12, if made of copper alloy, should, of course, be suitably corrosion-resistant; in certain cases a layer of bronze or brass may be used.

In any event the copper or copper alloy inner conductor layer should not make up more than from 7 to 20 percent of the total metal area since its presence is required only to resist the corrosion and possible ero sion due to the flowing of the cooling water through the inside or channel 11.

The cross sectional contour of the two layers may be the same; for example, they may both be circular or have some other suitable shape. The thicker outer layer made of aluminum or aluminum alloy may have a different external shape than that of the inner layer providing the interfaces of the two layers are the same shape. For example, FIG. 2 shows the outer layer as having a rectangular outer contour with rounded corners, although such outside contour could also be oval or polygonal.

In any event, there should be negligible electrical resistance between the interfaces of the two layers. This may be effected by making the conductor in such a manner that the interfaces are subjected to adequately high radial compression, possibly effected by rolling or pressing the aluminum portion onto a tubular core of copper or copper alloy.

However, it is preferred to make the multi-layer coil conductor by the hydrostatic extrusion process. This permits the production of the conductor in a relatively inexpensive manner and assures a substantially resistance-free integration of the interfaces. In this instance the connection is substantially metallurgical in character, having the effect of compression welding. Also, the differing conductor contours previously referred to, may be effected without difficulty. The hydrostatic extrusion process is well known, and anyone unfamiliar with it may refer to the combined teachings of British patent specification No. 1,240,154 and British patent specification No. 1,227,154.

FIG. I shows an example of the above. In this case a billet having a central copper tube I5 and an aluminum tube I6 concentric with the copper tube, have their upper ends sealed by an end piece 17, using an elastic sealing sleeve I8, this entire workpiece being placed in the high pressure cylinder 19 of a hydrostatic press. A mandrel 20 is positioned inside of the copper tube 15 and the bottom of the cylinder [9 contains a die 21 having a convergingly conical die orifice. The mandrel is supported by a tubular support 22 surrounding the combination to be extruded, and having ports 23. The hydrostatic pressure is provided by a ram [90 which fits the cylinder 19 and is moved downwardly to displace liquid, such as water, in the cylinder 19 between the ram 19a and the top of the support 22. With the development of hydrostatic pressure inside of the support 22, via the ports 23, the parts and 16 are simulta neously extruded through the die 21 to form the conductor shown by FIG. 2, for example.

An extrusion ratio at least exceeding 50, preferably exceeding lO0, and, for example, going up to 10,000, should be obtained. By this is meant the ratio between the diameter of the tubes 15 and 16 before and after the extrusion operation. The hydrostatic pressures would ordinarily be more than 15 kbar.

FIG. 3 schematically shows a crucible 24 containing a metal melt 25 heated by an induction coil 26 which surrounds the crucible 24. The coil 26 is wound from the multi-layer conductor of the present invention, such as the one illustrated by FIG. 2. Its terminal ends are connected with a suitable AC current supply 27 via connections 28 of the type providing for the introduction and removal of cooling water as indicated by the arrows shown in FIG. 3.

The crucible 24 and coil 26 are separable from each other so that a standard or copper or copper alloy tubular conductor coil may be used with the crucible if desired. It is to be understood that the crucible and coil may be standardized products of a furnace manufacturer, the difference between the coils being only that one may be in accord with the present invention and the other may be the standard prior art all-copper coil.

The above has the following advantages:

When designing crucible induction furnaces, for example, it is normally desirable to be able to calculate the electrical losses in advance, and such losses may be changed by redesigning, without reconstructing the furnace. It is desirable to be able to make a standard furnace, providing either a lower cost with incidental higher electrical losses or a higher furnace cost but having lower losses. A furnace buyer can then chose a design according to his own evaluation of permissible electrical losses, but if the buyer changes his mind, it is desirable to change the loss involved.

Therefore, the furnace of FIG. 3 and furnaces of other designs having separable crucibles and coils, may be standardized in construction, but providing for two alternative coils, one being the usual copper coil and the other being a coil according to the present invention, and the two kinds of coils may be given identical measurements so that they can be mounted in the furnace and connected to the electrical supply. In such a case a coil made according to the present invention will first of all be considerably less expensive than the copper coil, but on the other hand the electrical losses will be somewhat higher. For comparison, a coil made according to the present invention using an aluminumcopper combination, will involve losses about 25 percent higher than in the case of a conventional pure copper coil, but the manufacturing cost of the coil of this invention will be only about one-third that of the copper coil, this being based on the present cost of the two metals. By making the furnace with a standardized construction as to all components, a furnace buyer may have the choice between a high purchase price and lower electrical losses or a lower price while tolerating the somewhat higher electrical losses.

In the case of such a standardized furnace construction concept, the differences between a pure copper coil and a coil of the present invention using pure copper and pure aluminum and made as described hereto fore, in the case of a l2 ton lowfrequency current crucible furnace of conventional design are as follows:

Copper-Aluminum Copper Coil Coil Melting power (kW) I450 1447 Copper loss (kW) 543 603 Total loss (kW) 680 740 Input power (kW) I993 2050 Reactive power (kVAr) 959l 9570 Voltage (V) I237 1237 Current (A) 7918 7900 At current meta] costs, the cost of the pure copper coil is about three times higher than the cost of the cop per-aluminum coil of the present invention, both coils being physically the same in dimensions and construction What is claimed is:

1. An electric induction furnace having a crucible for containing a metal melt and an electric induction coil for inductively heating a melt in said crucible, said coil being a coiled tubular conductor through which a cooling liquid can be flowed; wherein the improvement comprises said conductor having inner and outer metal layers, said inner layer being a metal selected from the class consisting of copper and copper alloy, and the outer layer being some other metal and which is sub stantially free from copper, said other metal being a metal selected from the class consisting of aluminum and aluminum alloy, said layers having integrated interfaces substantially free from electrical resistance. 

1. AN ELECTRIC INTRODUCTION FURMACE HAVING A CRUCIBLE FOR CONTAINING A METAL MELT AND AN ELETRIC INDUCTION COIL FOR INDUCTIVELY HEATING A MELT IN SAID CRUCIBLE, SAID COIL BEING A COILED TUBULAR CONDUCTOR THROUGH WHICH A COOLING LIQUID CAN BE FLOWED, WHEREIN THE IMPROVEMENT COMPISES SAID CONDUCTOR HAVING INNER AND OUTER METAL LAYERS, SAID INNER LAYERS BEING A METALSELECTED FROM THE CLASS CONSISTING OF COPPER AND COPPER ALLOY, AND THE OUTER LAYER BEING SOME OTHER METAL AND WHICH IS SUBSTANTIALLY FREE COPPER, SAID OTHER METAL BEING A METAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM ALLOY, SAID LAYERS HAVING INTEGRATED INTERFACES SUBSTANTIALLY FREE FROM ELECTRICAL RESISTANCE. 